Velocity shear and current driven instability in a collisional F-region

摘要

We have studied how the presence of collisions affects the behavior ofinstabilities triggered by a combination of shears and parallel currents inthe ionosphere under a variety of ion to electron temperature ratios. To thisgoal we have numerically solved a kinetic dispersion relation, using arelaxation model to describe the effects of ion and electron collisions. Wehave compared our solutions to expressions derived in a fluid limit whichapplied only to large electron to ion temperature ratios. We have limited ourstudy to threshold conditions for the current density and the shears. We havestudied how the threshold varies as a function of the wave-vector angledirection and as a function of frequency. As expected, we have found that forlow frequencies and/or elevated ion to electron temperature ratios, thekinetic dispersion relation has to be used to evaluate the thresholdconditions. We have also found that ion velocity shears can significantlylower the field-aligned threshold current needed to trigger the instability,especially for wave-vectors close to the perpendicular to the magnetic field.However the current density and shear requirements remain significantlyhigher than if collisions are neglected. Therefore, for ionosphericF-region applications, the effect of collisions should be included inthe calculation of instabilities associated with horizontal shears in thevertical flow. Furthermore, in many situations of interest the kineticsolutions should be used instead of the fluid limit, in spite of the factthat the latter can be shown to produce qualitatively valid solutions.